Fiber-reinforced composite materials have been widely employed as excellent lightweight materials having high physical properties, because mechanical strength of matrix resins can be reinforced by fibers having high strength. In particular, a fiber-reinforced composite material including short fibers (discontinuous fibers) as the reinforcing fibers and a thermoplastic resin as the matrix resin has high physical properties and easy processability, so that in recent years its application fields have been widened, and its large developments have been expected.
However, when the reinforcing fibers in the matrix resin are discontinuous short fibers, there has been a problem that ends of the fibers are easily projected from a surface resin layer, compared to the case where long fibers are used as continuous fibers. Further, when the modulus of the fibers is high, or when the matrix resin used is soft, this phenomenon has been remarkable. For example, when the thermoplastic resin is used as the matrix resin, the phenomenon of the reinforcing fibers in an inside of the composite material, breaking through the resin layer existing on the surface more easily occurs, because it is soft, compared to a thermosetting resin.
In particular, this problem has been remarkable in the case where the surface resin layer is thin and further the reinforcing fibers are inorganic fibers such as rigid carbon fibers, or in the case of the composite material in which single fibers are kept in a fiber bundle form.
Furthermore, a press molding method is known as a method for efficiently producing the composite material. However, the surface resin layer becomes thin, compared to the other methods such as injection molding, so that this tends to be disadvantageous for the above-mentioned fiber projection. In addition, in the press molding, the reinforcing fibers used are included in the composite material in a strongly deformed state at the time of the press molding, thereby resulting in that relatively higher internal stress than in the other methods remains. For this reason, when the strength of the thin resin layer on the surface of the composite material decreases in this state for some reason, the remaining internal stress is released. There has been a problem that the above-mentioned phenomenon of the fibers breaking through the surface resin layer of the fiber-reinforced composite material more easily occurs.
Such a phenomenon caused by the internal stress of the material at the time of the press molding is generally called spring back. Then, in the fiber-reinforced composite material, a part of this spring back phenomenon has tended to come to the surface as the projection phenomenon of the reinforcing fibers to the composite body surface.
For example, as a specific composite material, a composite material obtained by press molding a fiber matrix structure containing discontinuous carbon fibers and a thermoplastic resin is disclosed in Patent Document 1. However, such a composite material has a problem that internal stress tends to remains in the reinforcing fibers although it is a composite material having high physical properties and excellent surface quality. For example, when the physical properties of the surface matrix resin deteriorate with the lapse of time or the like, the surface quality has tended to deteriorate.
[Patent Document 1] JP-A-2011-178890